Visiting http://net.ipcalf.com/ with Tails 0.22~rc1 displays the local IP address. Pretty scary...
TBB has disable that option at compile time and we should do the same.
Disable WebRTC (Closes: #6468).
On the long run, we want to do this at build time, just like the TorBrowser.
However, fiddling with the build options two days before a Tails release seems
crazy, so let's disable this with a pref for now.